Avoid loosing of the display mode value

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1200 matching lines @@
     ASSERT(node);
     if (isHTMLObjectElement(*node) || isHTMLEmbedElement(*node))
         toHTMLPlugInElement(node)->setNeedsWidgetUpdate(true);
 
     m_partUpdateSet.add(&object);
 }
 
 void FrameView::setDisplayMode(WebDisplayMode mode)
 {
     m_displayMode = mode;
-    m_frame->document()->mediaQueryAffectingValueChanged();
+    if (m_frame->document())
+        m_frame->document()->mediaQueryAffectingValueChanged();
 }
 
 void FrameView::setMediaType(const AtomicString& mediaType)
 {
     ASSERT(m_frame->document());
     m_frame->document()->mediaQueryAffectingValueChanged();
     m_mediaType = mediaType;
 }
 
 AtomicString FrameView::mediaType() const
@@ skipping 2828 matching lines @@
 {
     Settings* settings = frame().settings();
     if (!settings || !settings->rootLayerScrolls())
         return this;
 
     LayoutView* layoutView = this->layoutView();
     return layoutView ? layoutView->scrollableArea() : nullptr;
 }
 
 } // namespace blink